Hi Don,
It is possible for a hydraulic cylinder to pull in air and not leak out any
fluid.
Master cylinder would be more likely than a slave.
Usually visual inspection of either will show that they are old, leaking
etc.
How old are yours ?
You made it sound like they are newish. 
'usually' .............you are right, once it's fully bled and it's working,
it stays that way.
How is the clutch pedal push rod free play ? ...........clutch pedal should
move about an eighth inch 'free slop' at the top of the pedal - that's so it
is for sure not touching the clutch master cylinder piston.  
My hunches..............old parts. cheap parts. a very rare faulty new part,
or workmanship and bleeding, or some very weird / odd issue with the pipes
or the hose back there. 
If the van came to me freshly, I'd do visual inspection, check that free
play,
And if nothing turned up, bleed it very carefully.
I do not use pressure bleeders btw. I have one.  I just do it very carefully
with a manual method.  Once in a blue moon a pressure bleeder might get the
nth tiny bit of air out that I can't manually...........but that's very
rare. I bleed brakes and clutch hydraulic about 200 times a year or
something, and I haven't used my pressure bleeder rig in 7 years at least.
It's 'usually' details and workmanship, unless you have too worn parts. 
You might be doing everything right and have a real fluke situation.
But I'd do that -visual inspection, bleed very carefully. 

And of course the clutch itself comes to mind.
Cracked clutch slave bracket ?
I can't tell you how often people think it's fine to leave extra support
brackets off things ........i see it all the darn time. 
Scott
www.turbovans.com 
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Don Hanson
Sent: Thursday, January 17, 2008 4:12 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Clutch weirdness 84 standard trans ?

Thanks all.

  I guess I will take Ken's advice and do both clutch cylinders.  I do keep
my brake fluid fresh and I do have a power bleeder.  The intermittent poor
clutch was what had me scratching my head..it seems like you either have
air or not...not sometimes yes sometimes no...  But as I think about that,
the clutch slave IS all the way at the end of the line and I can visualize
it taking in some air then pushing it out the same leak again...sort of.
  Don Hanson